In a typical cellular wireless communication system, a radio access network (RAN) includes a plurality of base stations, each of which radiates to define one or more coverage areas such as a cell and cell sectors in which wireless communication devices (WCDs) can be served by the RAN and can thereby obtain connectivity with broader networks such as the public switched telephone network (PSTN) and the Internet.
A RAN will typically communicate with served WCDs according to an agreed air interface protocol, examples of which include CDMA (e.g., 1xRTT or 1xEV-DO), iDEN, WiMAX, LTE, GSM, HSDPA, and others now known or later developed. Communications in the direction from the RAN to WCDs define a “forward link”, while those in the direction from WCDs to the RAN define a “reverse link”.
Air interface communications in each coverage area may occur on one or more carrier frequencies, such as one frequency for forward link communications and another frequency for reverse link communications. Further, depending on the air interface protocol, the air interface may be divided into particular channels through a mechanism such as time division multiplexing, code division multiplexing, and/or frequency division multiplexing, for instance. By way of example, the forward link may define a pilot channel on which the RAN broadcasts a pilot signal for use by WCDs to detect and evaluate coverage, a paging channel on which the RAN may page WCDs, and one or more traffic channels on which the RAN transmits bearer data to WCDs that are actively engaged in calls (e.g., voice calls or data communications). The reverse link, on the other hand, may define an access channel on which WCDs may initiate calls and other communications, a power control channel on which WCDs may signal to the RAN to cause the RAN to adjust transmission power for traffic channel communications to the WCD, and one or more traffic channels on which WCDs may transmit bearer traffic to the RAN.
In general, when a WCD is actively engaged in a call in a coverage area of the RAN, the WCD may regularly monitor the strength (e.g., signal to noise ratio) of the pilot signal broadcast by the RAN in that coverage area and the strength of pilot signals broadcast by the RAN in other coverage areas extending to the WCD's position. If the WCD detects a pilot signal from another coverage area that is sufficiently stronger than the pilot signal of the WCD's current coverage area, the WCD may transmit a signal to the RAN to request a handoff of the call to the detected coverage area. The RAN may then determine if sufficient traffic channel resources exist in the new coverage area and, if so, may then assign traffic channel resources to the WCD for use in the new coverage area and direct the WCD to continue the call in the new coverage area.
Under some air interface protocols, a WCD may also be capable of engaging in a call actively in multiple coverage areas at once. In such an arrangement, the WCD may have an “active set” of coverage areas in which the WCD simultaneously exchanges bearer traffic with the RAN. In this arrangement, the WCD may regularly monitor the strength of pilot signals that it receives in each of its active set coverage areas as well as pilot signals broadcast in other coverage areas. If the WCD thereby detects a pilot signal from another coverage area that is sufficiently stronger than the weakest pilot signal of the coverage areas in the WCD's active set, the WCD may the signal to the RAN to request a handoff from that weakest coverage area to the newly detected coverage area. And the RAN may likewise assign traffic channel resources to the WCD in the newly detected coverage area and allow the call to continue with a revised active set.
Furthermore, while a WCD is actively engaged in a call, the WCD and RAN may engage in a power control process to help control the power level at which the RAN transmits bearer traffic to the WCD on a forward link traffic channel. A goal of this process is to keep the traffic channel transmission power at a level that is sufficiently high to allow the WCD to receive traffic communications from the RAN but not so high as to unduly interfere with other air interface communications, such as communications to and from other WCDs.
In an example traffic channel power control process, the WCD repeatedly measures the power level of traffic channel communications that the WCD is receiving from the RAN and compares the measured power level with a dynamically defined setpoint value. Based on each comparison, the WCD then sends a power control command, such as a Boolean value, to the RAN to cause the RAN to either increment or decrement the RAN's traffic channel transmission power. For instance, when the comparison shows that the received traffic channel power is greater than the setpoint, the WCD may transmit a power-down command to the RAN, and the RAN would responsively decrement its traffic channel transmission power. On the other hand, when the comparison shows that the received traffic channel power is less than the setpoint, the WCD may transmit a power-up command to the RAN, and the RAN would responsively increment its traffic channel transmission power. (In the event the received power equals the setpoint, the WCD may alternate between transmission of power-up and power-down commands to the RAN, to help maintain the transmission power at that level.)
In a system where a WCD operates with an active set, the WCD may apply this power control process for each coverage area in the WCD's active set, and the RAN may be arranged to adjust traffic channel transmission power to the WCD based on the power-control commands received for the group of active set coverage areas. For instance, the RAN may be arranged to increment its traffic channel transmission power in each active set coverage area if the WCD sends power-up commands in all of the active set coverage areas, but to decrement its traffic channel transmission power in each active set coverage area if the WCD sends a power-down command in any of the active set coverage areas.